1. Field of the Invention
This invention relates to a method of heat treating articles by immersing them in a fluidised bed.
2. Description of the Prior Art
It is known in the heat treatment of articles to provide a fluidised bed, which is formed of refractory articles, and means for supplying a gas, or gas/air mixture, which not only provides a sufficient velocity of gas or gas/air flow for fluidising the bed but which also provides within the bed the desired atmosphere for providing the required heat treatment process, it being understood that heat will also be supplied in any convenient manner. Examples of such heat treatment processes are carbo-nitriding, oxy-carbo-nitriding, carburizing, neutral annealing, natural stress relieving, and neutral hardening, the gas or gas/air mixture admitted to the bed in these cases having an appropriate composition to provide the correct atmosphere for the required treatment. The heat treatment of articles in a fluidised bed also includes processes in which paint or plastics material coatings are removed from painted or coated metal articles.
It is common practice, in constructing a fluidised bed, to make use of a porous bottom such as may be provided by a porous ceramic tile through which a gas/air mixture can be blown. It is also known to arrange that the gas and air shall be pre-mixed, externally of the fluidised bed, to provide a stoichiometric mixture which can then be ignited so as to provide a burning gaseous mixture which not only fluidises the bed but also heats the refractory particles of the bed. Ignition initially takes place just above the top of the bed and the flame front then moves downwardly into the bed. Such an arrangement does however present three problems. Firstly, the presence of a stoichiometric gas/air mixture outside the fluidised bed constitutes an explosion risk and fire hazard. Secondly, because a stoichiometric mixture is passing through the porous tile, the flame front will eventually reach or approach close to the tile and the temperatures to which the tile is then subjected may cause breakdown of the tile material or tile fixing material. Thirdly, the bed will be fluidised by the burning gases whereas in order to obtain good temperature control and optimum fluidising conditions it is desirable that the fuel input rate and the fluidising velocity should be independently variable.